In recent years, since an ink-jet recording system makes it possible to easily form images at lower cost, it has been widely applied to various printing and imaging fields such as photographic imaging, various types of printing, or special printing such as marking or color filters. Specifically, due to the development of ink-jet recording apparatuses capable of controlled ejection of minute ink droplets, ink whose color reproduction range, durability, and ejection adaptability have been improved, and special ink-jet paper whose ink absorbability, the color formation of colorants, and surface glossiness have been enhanced, whereby image quality is more and more approaching that of silver salt photography.
However, this image recording system necessitates special ink-jet paper and exhibits problems in which usable recording media are limited, and the cost of preparing the recording media is relatively high.
On the other hand, in an office environment, there has been greater demand for a new system capable of performing full color printing on various recording media at high speed without limitation of recording media (for example, using plain paper, coated paper, art paper, and dual-sided printing on plain paper).
Regarding ink-jet ink compositions, various studies have been conducted from several viewpoints, such as high-speed printing and excellent text reproduction on plain paper, without strike-through (being a phenomenon that printed ink goes through a printing medium and the text and/or image is seen on the reverse), feathering and bleeding of the image, and further, quick ink penetration for quick drying.
As one resulting type of ink-jet ink, a so-called water-based ink-jet ink has been widely employed, the water content of which ink is more than 50 weight %. However, in cases when image recording is conducted employing such water-based ink-jet ink onto copy paper for electrophotography or plain paper such as bond paper and medium quality paper, curling and cockling of plain paper become major drawbacks, adding to the problems of image strike-through and feathering caused by penetration of the ink.
Related to the above-cited problems, disclosed is an ink-jet recording method employing the ink, the penetration of which is enhanced by specific wetting time and the absorption coefficient to a recording medium of the Bristow method. (For example, please refer to Patent Document 1.) However, with this method, since the coloring material in the ink also permeates the interior of the plain paper simultaneously, a decrease of the image density and strike-through of the ink increase, resulting in a serious drawback of lack of readability of dual side printing.
Further, disclosed is an ink-jet ink containing a specific amide compound, a pyridine derivative, an imidazoline compound, or a urea compound as a curling reduction agent. (For example, please refer to Patent Document 2.) However, this method exhibits the problem of easy clogging nozzles of a recording head concurrent with ink drying.
Further, as a method to decrease the curling cited above, proposed is an ink-jet recording method in which curling is made more acceptable by providing a water containing solution to the rear side of the image printed surface. (For example, please refer to Patent Document 3.) However, this method exhibits drawbacks of not allowing dual sided printing and easily causing media jamming during conveyance due to decreased strength of the plain paper due to an increased amount of ink and the curling balancing solution.
Further, a solvent system ink which enables high speed printing was also studied as a substituent for a water-based ink. That is, by employing an oil-based ink (being a solvent system ink-jet ink) which contains a volatile solvent to raise drying efficiency, it is possible to print at higher rates on plain paper, because the ink penetrates into a recording medium very quickly and also dries quickly, so that curling of the recording medium does not result. However, the solvent system ink exhibits high permeability into plain paper, resulting in a big hurdle to surmount for dual sided printing, specifically onto plain paper due to poor text reproduction and strike-through of the recording medium.
Further, there is a prior art in which a black ink for a water-based ink-jet ink contains carbon black pigment dispersed in a dispersing medium of a non-polar insulating solvent. (Please refer, for example, to Patent Document 4.) Also, there is other prior art of a black ink which contains carbon black dispersed in a dispersing medium containing water as the main component, a water-soluble solvent and a polymer dispersing agent. (Please refer, for example, to Patent Documents 5-8.)
However, with these methods, it is impossible to stably maintain the carbon black in the ink in a dispersed state in the solvent. Therefore, the problem which is caused by flocculation of the carbon black particles in the ink, resulting in clogging of nozzle orifice of the recording head, is still not resolved.
For this reason, ink-jet inks, ejection property of which and decapping suitability are improved and which exhibit superiority in strike-through resistance, high text quality and curling characteristics, and ink-jet recording methods employing these inks, are disclosed. (For example, please refer to Patent Documents 9 and 10.)
Disclosed is the possibility to produce ink-jet inks featuring superiority in ejection stability and decapping performance, and also higher text quality of the formed image, strike-through resistance, and reduced curling characteristics of the printed recording material with these methods, in which a water-insoluble coloring material is dispersed in a dispersing medium, in which a solvent is incorporated in an amount of more than 50 but less than 90 weight % based on the total ink weight, and the water content is more than 10 but less than 45 weight %.
However, in the above-cited ink, the dispersion stability of a water-insoluble coloring material, specifically carbon black employed in the black ink, is poor. That is, the pigment particle diameter increases over time, and the particles in the ink separate from the liquid, resulting in clogging of the head orifices when the ink is ejected.                [Patent Document 1] Unexamined Japanese Patent Application Publication No. (hereinafter, referred to as JP-A) 10-316915        [Patent Document 2] JP-A 9-176538        [Patent Document 3] JP-A 10-272828        [Patent Document 4] JP-A 5-320551        [Patent Document 5] JP-A 10-60330        [Patent Document 6] JP-A 2003-73595        [Patent Document 7] JP-A 2001-348523        [Patent Document 8] JP-A 10-60328        [Patent Document 9] JP-A 2005-220296        [Patent Document 10] JP-A 2005-220297        